T-cells play a central role in virtually all types of immune responses. These cells trigger antibody production, initiate delayed-type hypersensitivity, generate killer cells, and in some instances, exert immunosuppressive effects. Our recent studies represent the first direct demonstration that these responses are not mediated by a single class of pluripotent T-cells. Rather, this class is divisible into several subclasses, each of which is equipped to mediate a limited range of immune functions, each distinguisable by expression of different surface Ly markers. Thus, cells capable of generating killer, helper, and suppressor function are likely to fall into distinct Ly ion subclasses. These findings open a new area of research which has important theoretical and practical implications. We are investigating the immunologic role of the above lymphocyte sets, and in particular the interactions among T-cell sets that regulate cellular and humoral immune responses. we are also continuing to define the contribution of cell surface Ly molecules to cytotoxic and helper responses, the influence of Ir H-linked genes upon differentiation of clones of helper and suppressor Ly sets. We are also using a new antisera, specific for "Ly5" as a probe for more precise definition of T-cell sets.